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Stockholders' Equity - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Jan. 08, 2018
Jan. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
May 05, 2016
Stockholders Equity Note [Line Items]            
Common stock, shares issued and sold     136,853,000 115,555,000    
Common stock, par value     $ 0.001 $ 0.001    
Proceeds from common stock sold     $ 85,036,000 $ 30,197,000 $ 22,121,000  
Preferred stock, shares issued     0 0    
Stockholders rights plan expiration date     Oct. 27, 2019      
Stockholder Rights Plan            
Stockholders Equity Note [Line Items]            
Minimum percentage of common stock acquired by stockholders     15.00%      
Designated preferred stock for future issuance     250,000      
Series C Junior Participating preferred stock            
Stockholders Equity Note [Line Items]            
Preferred stock, shares issued     0      
Amendment No. 2 | Cantor | Sales Agreement            
Stockholders Equity Note [Line Items]            
Maximum common stock offering price     $ 132,200,000      
Common stock registered for sale           $ 70,000,000
Amendment No. 3 | Cantor | Sales Agreement            
Stockholders Equity Note [Line Items]            
Proceeds from common stock sold     27,900,000 $ 30,300,000    
Maximum common stock offering price $ 70,000,000          
Common stock registered for sale     $ 41,600,000      
Percentage of proceeds payable as compensation to underwriter 2.00%          
Unsold shares of common stock, value $ 31,400,000          
Common stock, number of shares issued     4,200,000 11,000,000    
Common Stock            
Stockholders Equity Note [Line Items]            
Common stock, number of shares issued     18,202,000 10,986,000 3,526,000  
Common Stock | Underwritten Public Offering            
Stockholders Equity Note [Line Items]            
Common stock, shares issued and sold   14,030,000        
Common stock, par value   $ 0.001        
Common stock, sale of stock, price per share   $ 4.10        
Proceeds from common stock sold   $ 57,200,000